If a piezoelectric device is made by using a piezoelectric ceramic that undergoes only small changes in resonance frequency with temperature, the resulting piezoelectric device will have an advantage that the variations of resonance frequency characteristics with temperature are small. Thus, one of the desirable objectives in the field of piezoelectric devices is to decrease the rate of change of frequency of the piezoelectric ceramic with temperature. In particular, in the case where a piezoelectric ceramic is applied to a resonator, the change in oscillating frequency with temperature is largely dependent on the change in frequency of the piezoelectric ceramic with temperature. Thus, as the change in frequency of the piezoelectric ceramic with temperature becomes small, a higher precision resonator can be obtained.
One of the existing technologies for adjusting the resonance frequency temperature characteristics of piezoelectric ceramics related to the present invention is the technology disclosed in Japanese Unexamined Patent Application Publication No. 2001-39766 (Patent Document 1). Patent Document 1 discloses a piezoelectric resonator that utilizes thickness-shear vibrations and is composed of a piezoelectric ceramic containing a bismuth layer compound (CaBi4Ti4O15), in which the cut angle relative to the c axis of the piezoelectric ceramic is changed to adjust the rate of change of frequency with temperature.
However, it is necessary for the process to measure the orientation angle of the c axis lot-by-lot depending on the target rate of change of frequency with temperature according to the technology disclosed in Patent Document 1. Thus, the productivity is low. Moreover, a highly difficult process of measuring the angle is essential, which is problematic.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-39766